1. Field of the Invention
The present invention relates to a laser machining apparatus having functionality to detect the height position of the upper face of a workpiece held by a chuck table adapted to hold a workpiece.
2. Description of the Related Art
In a semiconductor device manufacturing process, devices such as ICs or LSIs are formed in a plurality of partitioned areas on the front face of an approximately disc-shaped semiconductor wafer. The plurality of areas are partitioned by a plurality of scheduled division lines called streets. The streets are arranged in a grid pattern. Then, the semiconductor wafer is cut along the streets, thus dividing the areas having devices formed therein and manufacturing individual semiconductor devices. On the other hand, an optical device wafer is formed by stacking a light-emitting layer made up of p- and n-type semiconductor layers, for example, on the front face of an approximately disc-shaped sapphire, silicon carbide, or gallium nitride substrate, and by forming light-emitting diodes, laser diodes or other optical devices in a plurality of areas partitioned by a plurality of streets. The streets are formed in a grid pattern. Then, the optical device wafer is cut along the streets, thus manufacturing individual optical devices.
As a method of dividing a semiconductor wafer, optical device wafer, or other type of wafer along streets, attempts have been made to use a laser machining method. This method employs a pulsed laser beam that penetrates a wafer and irradiates the pulsed laser beam to a condensing point inside an area to be divided. With this division method based on a laser machining method, a pulsed laser beam at a wavelength that penetrates a wafer is irradiated from one face of a wafer. The laser beam is irradiated to a condensing point inside the wafer. This continuously forms a modified layer inside the wafer along the streets. An external force is exerted on the streets whose strength has declined as a result of formation of the modified layer, thus dividing the workpiece (refer, for example, to Japanese Patent No. 3408805). If a modified layer is formed inside a workpiece along streets as described above, it is important to position a condensing point of a laser beam at a given depth from the upper face of the workpiece.
However, disc-shaped workpieces such as semiconductor wafers have undulations and vary in thickness. This makes it difficult to laser-machine a workpiece in a uniform manner. That is, if a modified layer is formed inside a wafer along streets, but if the wafer varies in thickness, a modified layer cannot be formed in a uniform manner at a given depth due to the relationship with the refractive index at the time of irradiation of a laser beam.
In order to solve the above problem, an interferometric height position detector is disclosed in Japanese Patent Laid-open Nos. 2011-122894 and 2012-2604. The interferometric height position detector detects the position of the upper face of a wafer held by a chuck table adapted to hold a workpiece based on the interference between light reflected from the upper face of the wafer and reference light. Further, a confocal optical height position detector is disclosed in Japanese Patent Laid-open No. 2009-63446. This detector detects the height position of the upper face of a wafer held by a chuck table adapted to hold a workpiece.